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Who Makes It Rain?

The artificial manipulation of the weather, known as weather modification, seems like something out of a science fiction novel, but more than 50 countries worldwide already participate in one type of weather modification known as cloud seeding.1 The U.S. Government Accountability Office predicts that in just the next decade, 40 U.S. states will experience some type of water shortage, which stems, in part, from droughts.2

The prospect of using cloud seeding to increase rainfall — its most popular usage — is an enticing one, but it comes along with a fair share of controversy as well. In the U.S., about $15 million is spent on cloud-seeding projects annually, which pales in comparison to the $100 million a year spent in China. Still, in the U.S., cloud seeding has grown by one-third in the last 10 years.3

It’s used in North Dakota, for instance, not only to promote rain but also to inhibit hail (and thereby reduce hail damage to crops). Cloud seeding is also sometimes used to clear fog.

In California, meanwhile, a mountain-top “cloud seeder” has been used to enhance rain and snow in efforts to fight drought. As its popularity grows, however, some are asking whether the practice is cost effective and whether it could end up having some negative effects on the weather, the latter of which is why some farmers in North Dakota are asking for the area’s cloud-seeding programs to end.

How Does Cloud Seeding Work?

There are a number of different ways that cloud seeding can work, but typically dry ice (frozen carbon dioxide) pellets or silver iodide are applied to certain clouds to modify their output. Seeding agents may be applied to clouds from the ground but, most often, aircraft are used to apply the materials to the clouds. This occurs either by releasing the seeding agent below the cloud into its updrafts or by dropping the seeds directly into the upper regions of the clouds. According to NDCMP — the North Dakota Cloud Modification Project:4

“In North Dakota, all seeding is done by aircraft. Base-seeding aircraft release seeding agent into updrafts from below the developing storm using a combination of wing-mounted ice nucleus generators and burn-in-place flares. Cloud-top seeding aircraft use ejectable flares and dry ice released directly into the supercooled cloud.”

There are several considerations as to which method is used. Direct injection works quicker, with results occurring almost immediately. However, NDCMP notes that this is costlier because it requires higher-performance aircraft capable of flying at higher altitudes and directly in-cloud. Updraft treatment, which is done at the cloud base, is an easier and less expensive method, but the results may take up to 30 minutes.

As for who’s in charge of deciding when to seed clouds, in North Dakota the program falls under the direction of the radar meteorologist. “In addition to weather conditions, a number of factors play a part in the decision-making process including safety criteria, radar information, pilot observations and aircraft instrument data,” according to NDCMP.5

North Dakota Reports Up to 10 Percent Increase in Rainfall Due to Cloud Seeding

Determining whether cloud seeding is effective poses a challenge, in part because many areas using the technology want all of the seedable clouds treated in order to reap the most potential benefits. In Wyoming, however, the Wyoming Weather Modification Pilot Program (WWMPP) conducted a randomized cloud-seeding program, which found the seeding increased snowfall by 5 percent to 15 percent under ideal seeding conditions.6

In North Dakota, meanwhile, NDCMP reports that cloud seeding produces an estimated 5 percent to 10 percent additional rainfall annually in the project area while reducing crop hail losses by 45 percent. As for costs, they say it only costs 16 cents per acre to enhance rain and suppress hail.7

The environmental effects are also reported as minimal, with NDCMP stating, “Cloud seeding agents, including silver iodide and dry ice, meet all National Environmental Policy Act (NEPA) regulations and are safe for the environment.” They further note that no environmentally harmful effects have been detected from cloud seeding with silver iodide, explaining:8

“The silver concentration in rainwater from a seeded storm is well below the acceptable environmental concentration of 50 micrograms per liter as set by the U.S. Public Health Service. Also, the concentration of iodine in iodized salt used for human consumption is far above the concentration found in rainwater from seeded clouds. Because silver iodide is such an effective ice nucleus, it is used in very small quantities.

Based on the average rate of silver iodide use in North Dakota each summer, it would take nearly 500 years for 1 gram of silver iodide (1/28th of an ounce) to be evenly spread out over an area equal to a full-sized basketball court!”

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